1. Field of the Invention
The invention relates to crosslinkable compositions of silane-crosslinking prepolymers, to processes for producing them, and to their use as adhesives and sealants, more particularly for the adhesive bonding of substrates.
2. Description of the Related Art
Polymer systems which possess reactive alkoxysilyl groups have a long history. On contact with water or atmospheric moisture, these alkoxysilane-terminated polymers are capable of undergoing condensation with one another even at room temperature, accompanied by elimination of the alkoxy groups. One of the most important applications of such materials is the production of adhesives, more particularly of elastic adhesive systems.
Adhesives based on alkoxysilane-crosslinking polymers, exhibit very good mechanical properties in the fully cured state, being capable not only of being highly elastic but also of possessing tensile strength. Another critical advantage of silane-crosslinking systems relative to numerous other adhesive and sealant technologies (to isocyanate-crosslinking systems, for example) is the toxicological unobjectionability of the prepolymers. Numerous applications prefer one-component systems (1K systems) which cure on contact with atmospheric moisture. A disadvantage of many systems conforming to the prior art is the low reactivity, which necessitates an aggressive catalysis. The mixtures in question therefore typically include considerable quantities of toxicologically objectionable tin catalysts.
An advantage here is the use of what are called α-silane-terminated prepolymers, which possess reactive alkoxysilyl groups connected through a methylene spacer to an adjacent urethane unit. This class of compound is highly reactive and requires neither tin catalysts nor strong acids or bases in order to achieve high cure rates on air contact. Commercially available α-silane-terminated prepolymers are GENIOSIL® STP-E10 or -E30 from Wacker-Chemie AG.
A disadvantage possessed by all silane-crosslinking adhesives, however, is that such adhesives fail to exhibit adequate adhesion to every material. This effects especially adhesion under humid conditions, more particularly the adhesion after cold-water storage. Adhesion problems often occur even with substrates such as coated steels, stainless steels, or anodized metals, or even glass, to which good or at least satisfactory adhesion can usually be achieved under dry conditions.
These problems occur to an increased degree with adhesives which exhibit a tensile strength of at least 1 MPa as measured by DIN EN 14293 and/or DIN EN 53504 after their curing. For adhesive seams with corresponding mechanical demands; therefore, these high-tensile adhesives impose exacting requirements in terms of adhesion, since on encountering high tensile forces they are required neither to rupture nor, of course, to undergo detachment from the substrate.
The adhesion profile of high-tensile adhesives can indeed be improved by addition of organofunctional silanes, in which case the combination of aminoalkyl-functional with glycidyloxyalkyl-functional alkoxysilanes is one of the combinations of adhesion promoters described as being especially advantageous for example in EP 1 179 571, EP 1 832 626, or EP 1 939 256, and their US equivalents U.S. Pat. No. 6,569,980 (B1), U.S. Pat. No. 7,781,525 (B2), or U.S. Pat. No. 8,076,439 (B2) respectively, the disclosures of which are incorporated in their entirety by reference herein. As described in WO 2007/131986, and its US equivalent US 2009/131591, the disclosures of which are incorporated in their entirety by reference herein, such combinations of silanes can be used to exert positive influence on other properties too such as the resilience, for example. The adhesion after cold-water storage, in contrast, cannot be sufficiently improved in this way.